Method and apparatus for measuring the electrical resistivity of geologic formations through metal drill pipe or casing

Abstract

The resistivity of a geologic formation is measure through drillpipe or casing by applying a low frequency bipolar voltage to the casing and to a ground electrode and traversing the casing with a tool having contractors which are connected to a differential amplifier to detect a differential voltage within the casing caused by current leaving the casing into the formation. The differential amplifier is connected to a unity gain inverter and a switching arrangement for alternately reversing the polarity of the connection of input voltages to the differential amplifier. A switching network alternately connects the outputs of the differential amplifier and the output of the inverter to the input of a low pass filter, and a clock is provided for driving the network in synchronization. The casing and formation resistivities may be measured by applying a voltage along a predetermined length of casing between two contractors which are spaced apart farther and on opposite sides of the differential amplifier pickup contactors. The voltage applied across the formation may be applied to the casing at the earth's surface or downhole adjacent the points of measurement of casing resistivity and formation resistivity.

What we claim is: 
     
       1. A method for measuring selected electrical properties of a subterranean formation into which at least one wellbore has been drilled and is provided with a tubular metal conduit extending within said wellbore, said method comprising the steps of: providing means insertable in said wellbore including a first set of spaced apart contact means for contacting said conduit, a current source connectable to said first set of contact means, a second set of spaced apart contact means for contacting said conduit between said first set of contact means, and means for measuring a voltage between said second set of contact means,   providing a source of electrical potential between said conduit and an electrode connected to said formation;   determining the resistivity of at least selected portions of said conduit by applying current from said current source to said first set of contact means and measuring the flow of current and the voltage differential across a selected portion of said conduit between said second set of contact means;   applying an electrical potential through said formation between said electrode and said conduit; and   measuring the voltage differential between one of said sets of contact means at a selected portion of said conduit, and determining the current flowing along said conduit based on the resistivity of said selected portion of said conduit to determine the current flow through said formation at a selected point on said conduit.   
     
     
       2. The method set forth in claim 1 including the step of: traversing said sets of contact means along said conduit and determining the resistivity of said conduit and the current flowing to said formation over a selected portion of said conduit.   
     
     
       3. The method set forth in claim 1 wherein: said source of electrical potential is connected to said conduit at a point near the earth's surface.   
     
     
       4. The method set forth in claim 1 wherein: said source of electrical potential is connected to said conduit at a selected point on said conduit below the earth's surface.   
     
     
       5. The method set forth in claim 4 wherein: said source of electrical potential is connected to said conduit through one of said contact means.   
     
     
       6. The method set forth in claim 1 wherein: said current source is a bipolar current source.   
     
     
       7. The method set forth in claim 6 wherein: said current source is a switched DC current source.   
     
     
       8. A method for determining electrical characteristics of an earth formation in which a wellbore has been formed and is provided with a tubular metal casing extending within said wellbore, said method comprising the steps of: providing an electrical source for application to selected incremental portions of said casing;   providing an electrical source for application to said formation through said casing and an electrode spaced from said casing;   measuring current flow and voltage differential along selected incremental portions of said casing and determining the resistivity of said selected incremental portions of said casing, respectively;   applying said electrical source to said formation through said casing and measuring voltage differentials along said selected incremental portions of said casing;   determining current flow along respective ones of said selected incremental portions of said casing based on measured voltage differentials and resistivity of said incremental portions of said casing, respectively; and   determining current flow through selected intervals of said formation by the difference between current flow through said casing at one incremental portion of said casing and another incremental portion of said casing.   
     
     
       9. The method set forth in claim 8 including the step of: determining the resistivity of a selected interval of said formation by the current flow through said selected interval and the voltage potential applied to said formation through said casing and said electrode.   
     
     
       10. The method set forth in claim 8 wherein: said electrical source is applied to said casing at a selected point in said casing below the earth's surface.   
     
     
       11. The method set forth in claim 8 wherein: said electrical source is applied to said casing at a point near the earth's surface.   
     
     
       12. A system for determining the resistivity of an earth formation by measuring voltage differentials along a metal conduit penetrating said formation, said system comprising: an electrical potential source having a first electrode, means for electrically connecting said first electrode to said conduit and a second electrode connected to said formation at a distance from said conduit;   a first set of spaced apart contactors for detecting voltages between first and second points in said conduit;   a differential amplifier having input conductors coupled to said first set of contactors, respectively, and having an output conductor;   an inverter circuit having an input conductor to said differential amplifier output conductor and having an inverter output conductor;   switching means having a first input conductor connected to said differential amplifier output conductor, a second input conductor connected to said inverter output conductor, a control input conductor and a switching means output conductor, said switching means being operable to alternately couple said first and second input conductors to said switching means output conductor; and   signal averaging means having an input conductor connected to the said switching means output conductor and an averaging means output conductor for conducting a signal indicating the differential voltage along said conduit between said first set of contactors.   
     
     
       13. The system set forth in claim 12 including: a second set of spaced apart contactors for conducting current through a portion of said conduit including said first and second points, and a current source for causing a current of known magnitude to flow through said conduit between said first and second points.   
     
     
       14. The system set forth in claim 13 including: means for connecting said potential source to said conduit at a point in said formation.   
     
     
       15. The system set forth in claim 14 wherein: said means for connecting said potential source to said conduit includes one of said contactors.   
     
     
       16. The system set forth in claim 12 including: means for connecting said potential source to said conduit near the earth's surface.   
     
     
       17. The system set forth in claim 13 including: clock means having an output conductor coupled to said current source and said switching means for causing said current source to alternate the polarity of current applied to said conduit in synchronization with the alternate coupling of said switching means first and second input conductors to said switching means output conductor.   
     
     
       18. The system set forth in claim 12 wherein: said signal averaging means comprise a low pass filter.   
     
     
       19. A system for determining the resistivity of an earth formation into which a wellbore has been formed and is provided with an elongated conduit extending within said wellbore, said system comprising: means for disposition in said wellbore for determining the electrical resistance of selected portions of said conduit including a first set of spaced apart contactors for engagement with said conduit, means connected to said first set of contactors comprising circuit means including a differential amplifier for measuring a voltage differential along said conduit between said first set of contactors, a second set of contactors disposed spaced apart such that said first set of contactors is between said contactors of said second set, a source of current for connection to said second set of contactors for causing a current flow of known magnitude through said conduit between said contactors of said first set;   an electrical potential source including means for connecting said potential source to said conduit and means for connecting said potential source to an electrode in electrically conductive contact with said formation at a point spaced from said wellbore; and   means for measuring the voltage applied to said formation between said conduit and said electrode.   
     
     
       20. The system set forth in claim 19, including: means for selectively connecting said potential source to said conduit at the earth's surface and at a point in said conduit below the earth's surface, respectively.   
     
     
       21. The system set forth in claim 19, wherein: said differential amplifier includes input conductors coupled to said first set of contactors, respectively, and having an amplifier output conductor and said circuit means includes;   an inverter circuit having an input conductor connected to said amplifier output conductor and having an inverter output conductor;   switching means having a first input conductor connected to said amplifier output conductor, a second input conductor and a switching means output conductor, said switching means output conductor being alternately coupled to said first and second input conductors;   a filter having a filter input conductor connected to said switching means output conductor and a filter output conductor; and   clock means having a clock output conductor coupled to said current source to alternate the polarity of current applied to said conduit in synchronization with the alternate coupling of said switching means first and second conductors to said switching means output conductor.   
     
     
       22. Apparatus for measuring voltage differentials along a metal conduit comprising: a set of spaced apart contactors for contacting first and second points on said conduit;   a current source and means for connecting said current source to said contactors, respectively;   a differential amplifier having input conductors coupled to said contactors, respectively, and having an amplifier output conductor;   an inverter circuit having an input conductor connected to said amplifier output conductor and having an inverter output conductor;   switching means having a first input conductor connected to said amplifier output conductor, a second input conductor connected to said inverter output conductor, a control input conductor and a switching means output conductor, said switching means output conductor being alternately coupled to said first and second input conductors;   signal averaging means having an input conductor connected to said switching means output conductor and an averaging means output conductor; and   clock means having an output conductor coupled to said current source and said switching means for causing said current source to alternate the polarity of current applied to said conduit in synchronization with the alternate coupling of said switching means first and second input conductors to said switching means output conductor to provide a signal on said switching means output conductor indicating the differential voltage along said conduit between said contactors.